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Subject[PATCH 03/12] rwsem: let RWSEM_WAITING_BIAS represent any number of waiting threads
Date
Previously each waiting thread added a bias of RWSEM_WAITING_BIAS. With this
change, the bias is added only once when the wait list is non-empty.

This has a few nice properties which will be used in following changes:
- when the mutex is held and the waiter list is known to be non-empty,
count < RWSEM_WAITING_BIAS <=> there is an active writer on that sem
- count == RWSEM_WAITING_BIAS <=> there are waiting threads and no
active readers/writers on that sem

Signed-off-by: Michel Lespinasse <walken@google.com>
---
lib/rwsem.c | 23 +++++++++++++++--------
1 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/lib/rwsem.c b/lib/rwsem.c
index caee9b7..0fb6e38 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -52,7 +52,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
struct rwsem_waiter *waiter;
struct task_struct *tsk;
struct list_head *next;
- signed long oldcount, woken, loop;
+ signed long oldcount, woken, loop, adjustment;

waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
@@ -68,9 +68,12 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
* write lock. However, we only only wake someone up
* if we can transition the active part of the count from 0 -> 1
*/
+ adjustment = RWSEM_ACTIVE_WRITE_BIAS;
+ if (waiter->list.next == &sem->wait_list)
+ adjustment -= RWSEM_WAITING_BIAS;
+
retry_writer:
- oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
- - RWSEM_ACTIVE_BIAS;
+ oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
if (oldcount & RWSEM_ACTIVE_MASK)
/* Someone grabbed the sem already */
goto undo_writer;
@@ -106,7 +109,10 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
} while (waiter->flags & RWSEM_WAITING_FOR_READ);

loop = woken;
- woken *= RWSEM_ACTIVE_BIAS - RWSEM_WAITING_BIAS;
+ woken *= RWSEM_ACTIVE_READ_BIAS;
+ if (waiter->flags & RWSEM_WAITING_FOR_READ)
+ /* hit end of list above */
+ woken -= RWSEM_WAITING_BIAS;

retry_readers:
oldcount = rwsem_atomic_update(woken, sem) - woken;
@@ -133,7 +139,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)

/* undo the change to count, but check for a transition 1->0 */
undo_writer:
- if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
+ if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK)
goto out;
goto retry_writer;

@@ -160,6 +166,8 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
waiter->task = tsk;
get_task_struct(tsk);

+ if (list_empty(&sem->wait_list))
+ adjustment += RWSEM_WAITING_BIAS;
list_add_tail(&waiter->list, &sem->wait_list);

/* we're now waiting on the lock, but no longer actively read-locking */
@@ -193,8 +201,7 @@ rwsem_down_read_failed(struct rw_semaphore *sem)
struct rwsem_waiter waiter;

waiter.flags = RWSEM_WAITING_FOR_READ;
- rwsem_down_failed_common(sem, &waiter,
- RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS);
+ rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_READ_BIAS);
return sem;
}

@@ -207,7 +214,7 @@ rwsem_down_write_failed(struct rw_semaphore *sem)
struct rwsem_waiter waiter;

waiter.flags = RWSEM_WAITING_FOR_WRITE;
- rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS);
+ rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_WRITE_BIAS);

return sem;
}
--
1.7.0.1


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